Cascaded cure approach to fabricate highly tensile silicon nitride films

ABSTRACT

A highly tensile dielectric layer is generated on a heat sensitive substrate while not exceeding thermal budget constraints. Cascaded ultraviolet (UV) irradiation is used to produce highly tensile films to be used, for example, in strained NMOS transistor architectures. Successive UV radiation of equal or shorter wavelengths with variable intensity and duration selectively breaks bonds in the Si—N matrix and minimizes shrinkage and film relaxation. Higher tensile stress than a non-cascaded approach may be obtained.

FIELD OF THE INVENTION

This invention relates to electronic devices and associated fabricationprocesses. More specifically, the invention relates to highly tensiledielectric films on heat sensitive substrates formed by UV curing, forexample implemented in strained transistor architecture for NMOS devicesin which a highly tensile silicon nitride capping layer is provided onthe source and drain regions to induce tensile strain in the NMOSchannel region.

BACKGROUND OF THE INVENTION

As transistors are scaled to smaller dimensions there is a need forhigher switching speeds. One solution to increase transistor speed is tostrain the silicon in the channel. Adding strain to the silicon latticestructure is believed to promote higher electron and hole mobilities,which increase transistor drain current and device performance.

One explanation for an increase in strain to improve device performanceis a change in electron symmetry. When a silicon lattice is undertensile strain, its physical symmetry is broken, and with it theelectronic symmetry. The lowest energy level of the conduction band issplit, with two of the six original states dropping to a lower energylevel and four rising to a higher energy level. This renders it moredifficult for the electrons to be ‘scattered’ between the lowest energystates by a phonon, because there are only two states to occupy.Whenever electrons scatter, their motion is randomized. Reducing scatterincreases the average distance an electron can travel before it isknocked off course, increasing its average velocity in the conductiondirection. Also, distorting the lattice through tensile strain candistort the electron-lattice interaction in a way that reduces theelectron's effective mass, a measure of how much it will accelerate in agiven field. As a result, electron transport properties like mobilityand velocity are improved and channel drive current for a given devicedesign is increased in a strained silicon channel, leading to improvedtransistor performance.

Transistor strain has been generated in NMOS devices by using a highlytensile post-salicide silicon nitride capping layer on the source anddrain regions. The stress from this capping layer is uniaxiallytransferred to the NMOS channel through the source-drain regions tocreate tensile strain in the NMOS channel. For example, a 1000 Å siliconnitride capping layer with a stress of 1 GPa has been shown to provide a10% NMOS I_(DSAT) gain (saturation drain current) from tensile channelstrain (Ghani, et al., A 90 nm High Volume Manufacturing LogicTechnology Featuring Novel 45 nm Gate Length Strained Silicon CMOSTransistors, IEEE (2003), incorporated by reference herein in itsentirety for all purposes). However, a tensile stress in excess of 1E10dynes/cm² is necessary for optimal results.

A highly tensile silicon nitride capping layer may be deposited using athermal CVD process, e.g., LPCVD. However, these processes generallyrequire temperatures of greater than 500° C. to remove hydrogen from thecapping layer to induce tensile stress in the capping layers, and atthese higher temperatures the underlying NiSi (silicide/salicide)substrate for the capping layer undergoes a phase transformation thatincreases its resistivity. NiSi structures require temperature of lessthan about 400° C. to avoid the resistivity increase. NiPtSi gateelectrodes has better thermal stability then NiSi, but even NiPtSistructures still require temperatures of less than about 480° C. Hence,thermal budget constraints for future transistor architectures requirethe films to be deposited at temperatures below than about 480° C.,preferably below than about 400° C. A lower temperature thermal annealmay be used, for example, one in which the anneal temperature does notexceed 480° C. However, the duration of a thermal anneal process at thattemperature that is necessary to obtain the benefit (e.g., about 2hours) is not economically viable, and neither is the stress achievedsufficiently high.

Accordingly, fabrication processes for generating greater NMOStransistor channel strain are desirable.

SUMMARY OF THE INVENTION

The present invention addresses this need by providing a processingtechnique to generate a highly tensile dielectric layer on a heatsensitive substrate while not exceeding thermal budget constraints.Cascaded ultraviolet (UV) irradiation is used to produce highly tensilefilms to be used, for example, in strained NMOS transistorarchitectures. Successive UV radiation of equal or shorter wavelengthswith variable intensity and duration selectively breaks bonds in the SiNfilm and minimizes shrinkage and film relaxation. Higher tensile stressthan a non-cascaded approach may be obtained.

Ultraviolet (UV) irradiation is used to produce highly tensile films tobe used, for example, in strained NMOS transistor architectures. UVcuring of as-deposited PECVD silicon nitride films, for example, hasbeen shown to bring about significant hydrogen removal and shrinkage,resulting in tremendous stress changes. Films with stresses of at least1.65 GPa have been successfully produced using this technique at atemperature of 480° C. Even higher stresses are in demand, since deviceperformance continues to increase with stress. The methods of thepresent invention create greater stresses by varying UV wavelengths,intensity, and duration to target specific bonds in the SiN film

In one aspect, the present invention pertains to a method of fabricatinga highly tensile dielectric layer on a heat sensitive substrate. Themethod includes providing the heat sensitive substrate, depositing adielectric layer on the substrate, exposing the dielectric layer UVradiation of a first wavelength or wavelength range at a temperatureless than about 480° C., and repeating the exposure one or more timeswith shorter wavelengths or wavelength ranges in successive exposures.Thus, each exposure operation may apply UV radiation of a particularwavelength or wavelength range, and a mixture of specific wavelength orwavelength ranges may be used in a sequence. Each wavelength orwavelength range is at most equal to or shorter than the previouswavelength or range. The cascaded approach results in a higher tensilestress in the dielectric film than the non-cascaded approach. Thedielectric film may be a silicon nitride, a silicon carbide,oxygen-doped silicon carbide, nitrogen-doped silicon carbide, siliconboron nitride, silicon boron carbide, silicon oxide, and combinationsthereof. In certain embodiments, the dielectric film is silicon nitrideor a doped silicon nitride capping layer on the source and drain regionsof an NMOS device.

In certain embodiments, the first wavelength or wavelength range mayinclude only wavelengths longer than about 300 nm, preferably about300-800 nm. After the first UV exposure, subsequent exposures havingshorter wavelengths are conducted. In one example, the UV exposureoperation may be repeated three times. A first repeat exposure may havea wavelength within or range of about 295-800 nm; a second range mayhave a wavelength within or range of about 280-800 nm; and a third rangemay have a wavelength within or range of about 225-800 nm. In certainembodiments, the substrate may be exposed only to UV radiation having awavelength longer than about 200 nm, or 225 nm, or preferably longerthan about 250 nm. In some cases, depending on the film being cured andrate of cure, a final stage cure wherein the film is exposed to all theavailable wavelengths, even if the wavelength is shorter than thecut-off in the previous stage from the broadband source (or specificwavelngths like 222 nm, 172 nm etc. if using an excimer lamp) may beused.

In certain embodiments, a long pass filter is used to narrow thewavelength ranges of a broadband UV source, which may emit radiationfrom about 170 nm to the infrared region (about 1-10 micron). A longpass filter may be used to attenuate, or reduce, intensity of UVwavelengths passing through to filter those that are shorter than acertain cut-off wavelength. A long pass filter may be a number ofdielectric layers with optical properties and thicknesses tuned tocertain UV wavelengths. A long pass filter may be used with a short passfilter to create a bandpass filter that only allows UV radiation ofcertain wavelength range to pass. A bandpass filter may pass radiationof a broad range, such as 300 nm to 800 nm, or a narrow range, such as250 nm to 260 nm. The wavelengths not transmitted are either reflectedor absorbed by the filter material.

In addition, or instead of, using a broadband UV source, a singularwavelength source such as a laser may be used. An excimer laser can emitUV radiation in specific wavelengths depending on the molecule used forthe electrical stimulation. For example, using Xe₂*, KrCl*, KrF, XeBrand XeCl, UV radiation having a wavelength of 172 nm, 222 nm, 248 nm,282 nm and 308 nm, respectively can be generated. In some cases, theradiation emitted by the excimer laser is scanned across the face of thesubstrate to completely expose the entire substrate. In other cases,multiple lasers may be used to form a pattern, which may be alsoscanned. Also, excimer lamps may also be used, wherein the emitted UVwavelength is quasi-monochromatic. A quasi-monochromatic wavelength is aspectrum having a defined peak and a defined range for full width athalf maximum (FWHM). For example, a quasi-monochromatic spectrum mayhave a peak at 222 nm with a FWHM of 10 nm.

In certain embodiments, the methods of the present invention may employa multi-station chamber. The stations in the chamber include apparatusfor exposing the substrate to one or more UV radiation wavelengths or awavelength range. The substrate may be transferred sequentially fromstation to station and be exposed to differing UV wavelengths,intensities, and for different durations. An example chamber may havefour stations, each with one or more UV radiation sources and,optionally, one or more filters.

By utilizing this cascaded approach, very high tensile stresses may beachieved. The tensile stress may be higher than 1.5E10 dynes/cm² or 1.5Gigapascal (GPa), preferably higher than 1.65E10 dynes/cm² or 1.65 GPa,or even more preferably higher than 1.8 GPa.

In certain embodiments, the present invention pertains to a method offabricating a highly tensile dielectric layer by providing a heatsensitive substrate, depositing a dielectric layer on the substrate, andexposing the dielectric layer to ultraviolet radiation having awavelength or wavelength range longer than about 250 nm. In theseembodiments, the tensile stress induced may be higher than the stressesachieved with ultraviolet radiations that include wavelengths shorterthan about 250 nm. The exposure may be repeated one or more times withUV radiation of shorter wavelengths with successive exposures, but atall times longer than 250 nm.

These and other aspects and advantages of the invention are describedfurther below and with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a plot of absorption spectra for a silicon nitride filmbefore and after UV curing for 10 minutes.

FIG. 1B shows a plot of absorption spectrum of an as deposited siliconnitride film with very little hydrogen in it.

FIG. 2A is a plot of the increase in tensile stress with UV curing timefor PECVD silicon nitride films using different radiation sources.

FIG. 2B is a plot of the shrinkage with UV curing time for PECVD siliconnitride films using different radiation sources.

FIG. 3A is a plot of the change in tensile stress with curing time forPECVD silicon nitride films comparing UV cure using various sources andhigh temperature anneal.

FIG. 3B is a plot of the change in shrinkage with curing time for PECVDsilicon nitride films comparing UV cure using various sources and hightemperature anneal.

FIG. 4 depicts a process flow diagram for a method of fabricating a hightensile stress dielectric film in accordance with an embodiment of thepresent invention.

FIG. 5 depicts a process flow diagram for a method of fabricating a hightensile stress dielectric film in accordance with an embodiment of thepresent invention.

FIG. 6 depicts a simple transistor architecture in accordance with anembodiment of the present invention.

FIGS. 7A and 7B are simple block diagrams depicting a chamber configuredfor implementing the deposition of dielectric films in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to specific embodiments of theinvention. Examples of the specific embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to such specific embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

Introduction

The present invention provides a processing technique to generate ahighly tensile dielectric layer on a heat sensitive substrate while notexceeding thermal budget constraints. Ultraviolet (UV) radiation is usedto produce highly tensile films to be used, for example, in strainedNMOS transistor architectures. UV curing of as-deposited PECVD siliconnitride films using broadband UV sources, for example, has been shown toproduce films with stresses of at least 1.65 E10 dynes/cm², or 1.65 GPa.Other dielectric capping layer film materials show similar results. Intransistor implementations, the stress from a capping layer composed ofsuch a film is uniaxially transferred to the NMOS channel through thesource-drain regions to create tensile strain in NMOS channel. Thearchitecture has been developed for 90 nm logic technology on 300 mmsubstrates, although it is not so limited in application. As thetechnology nodes advances to 65 nm and smaller, higher stresses arerequired to maintain the same device performance as the films becomesthinner. Additionally, device performance continues to improve withincreased stresses. Thus, greater stresses continue to be desired.

In the conventional process, broadband UV radiation is used, havingwavelengths range from less than 200 nm all the way to infrared.Absorption of the radiation in the UV region, followed by bond-breakingand H₂ removal, and subsequent stress generation has been the mechanism.The broadband UV radiation breaks the Si—H, N—H, and Si—N bonds in thefilm. After the Si—H and N—H bonds are broken and the hydrogen diffusesout of the film, the dangling silicon and nitrogen bonds form a SiNnetwork. After Si—N bonds are broken, the bonds reform in a preferredenergy pathway.

Significant tensile stress is generated within the first few minutes ofcure (<10 min) accompanied by high shrinkage. With increasing cure time,the rate of stress generation drops off significantly, and continuedcuring brings about a stress reversal (stress starts decreasing).Although not intending to be bound by this theory, the reason for thestress saturation and reversal may be two-fold: first, decreasinghydrogen concentration in the film and increased compaction (due toshrinkage) makes it difficult to continue evolving hydrogen from thefilm. Hydrogen diffusion through the film depends on many factors,including time, film temperature, micro-structure of the film, andconcentration of hydrogen in the film. As the film shrinks, the hydrogenevolution decreases to a point where further bond breaking does notincrease the SiN network because the hydrogen is unable to diffuse out.While a decrease in hydrogen diffusion accounts for stress saturation,the stress reduction is likely due to a relaxation phenomenon.

The stress relaxation phenomenon may be caused by the reformation ofSi—N bonds in a lower energy state. As the film cures, the number ofSi—H and N—H bonds decreases, leaving more Si—N in the film. Since thefilm is exposed to a broadband source, radiation in the shorterwavelengths (higher energy) continues to break the Si—N backbone. Whilethese bonds immediately reform, they reform in a less-strainedconfiguration. Hence it is not possible to generate higher stresses bycuring longer as more configurations are reformed in a less-strainedmanner.

Film stress may be increased if breaking of the Si—N backbone isminimized while continuing to break hydrogen bonds and evolve thehydrogen. The present invention reduces breaking of the Si—N backbone byexploiting the difference in absorption spectrum of the Si—H, N—H, andSi—N bonds. FIG. 1A shows the absorption spectra a high tensile nitridefilm as-deposited, as line 101. Upon curing the film for 10 minutes, theabsorption curve shifts to the left to the shorter wavelengths (shown asline 103), with absorption terminating at about 270 nm, effectivelyterminating at about 250 nm, where the absorption is only about 3% ofthat at 190 nm. FIG. 1B shows the absorption spectra (105) for a SiNfilm deposited at high temperature on a CVD reactor. As discussed above,high temperature CVD SiN film has very little hydrogen in it. FIG. 1Bshows that the absorption terminates at about 225 nm for such a film.Because the high temperature SiN film has mostly only Si—N bonds, itfollows that Si—N absorbs radiation only lower than 225 nm. FIG. 1A alsosupports this conclusion, as evidenced by Si—H and N—H bonds' absorptionof UV radiation between curves 101 and 105, estimated to be above 225nm, preferably 225-350 nm.

In one aspect, the present invention involves generating tensile stressin dielectric films by treating the films with UV radiation of shorterwavelengths or wavelength ranges in successive exposures, whilemaintaining all UV radiation at a wavelength of a cut-off value andlonger. In one embodiment, the present invention involves generatingtensile stress by treating a silicon nitride film with UV radiation at awavelength of 225 nm and longer. The cut-off limits on the wavelengthsare guides to determine the appropriate wavelength to use. The siliconnitride film has a cut-off wavelength of 225 nm. Other variants of thesame SiN film, or other films such as SiC, SiCN, SiBN, etc. will eachhave their own cut-off limits.

Post-Deposition UV Treatment of Dielectric Layer

The post-deposition UV treatment, also referred to as UV curing,technique of the invention generates a highly tensile dielectric layeron a heat sensitive substrate without thermally degrading the substrate.In a particular implementation, source/drain salicide capping layerstress in NMOS transistors is increased without thermally degrading thesalicide. The technique involves post-deposition UV treatment of acapping layer film on a heat sensitive substrate, e.g., a silicide (orsalicide) such as NiSi. In a specific embodiment, the capping layer iscomposed of silicon nitride (SiN or Si₃N₄) deposited by PECVD and theinvention is primarily described herein with respect to this embodiment.In alternative embodiments, however, the capping layer may be depositedby other thermal CVD process, e.g., low pressure CVD (LPCVD),atmospheric pressure CVD (APCVD) or other suitable techniques such asspin-coating, print-coating, and dip-coating, and be composed of otherdielectric materials including silicon carbide (SiC), oxygen-dopedsilicon carbide (SiCO), nitrogen-doped silicon carbide (SiCN), siliconboron nitride (SiBN), silicon boron carbide (SiBC), and silicon oxide(SiO or SiO₂), and the invention should be understood to apply to thesecapping layer materials as well. Further the present invention alsoapplies to other situations where greater stresses are desirable and maybe induced by tailoring successive UV exposures at a wavelength known tobreak particular bonds to form a desired cured structure.

PECVD films in general contain a considerable amount of hydrogen. Forexample, PECVD silicon nitride films contain generally contain about15-30% hydrogen in the form of Si—H and N—H bonds. Again, while theinvention is not limited by this theory, it is generally accepted thatirreversible tensile stress develops in PECVD films from the reductionof the amount of hydrogen in the film, and due to shrinkage of voids.The loss of hydrogen and shrinkage of voids result in a volume reductionin the film. But the constraint of the substrate prevents any lateralshrinkage, thus imposing tensile strains in the film. The change inhydrogen concentration has been shown to be proportional to theirreversible stress change. Thermal annealing at temperatures in excessof 500° C., e.g., 600° C., are also known to remove the hydrogen fromsuch a film (silicon nitride) by providing sufficient energy to attainthe right bonding configuration and stoichiometry, in particular theremoval of H and formation of extended Si—N bonds. These temperatures,however, exceed the thermal budget constraints, leading to, for example,higher resistivity of NiSi. In some cases, annealing at thesetemperatures may adversely affect the partially fabricated circuits andfilms.

It has now been found that UV irradiation is a source of energy thatmakes compressive films tensile and tensile films even more tensile. Itis believed that the photons from the UV source for example, a “H bulb”of a Hg lamp having a nominal wavelength from about 150 nm to 800 nm andan intensity of between 1 μW/cm² and 10 W/cm², provide sufficient energyto break most H-bonds in a dielectric film, e.g., for a silicon nitridefilm, the Si—H and N—H bonds. UV irradiation has a penetration depthsufficient to treat a film of full thickness, for example, between about50 and 30,000 Å, e.g., about 300-1500, such as 700 Å. A following gas,such as He, Argon or N₂ may be used as a purging gas during UV curing toremove evolved hydrogen. Alternatively or in addition, the chamber maybe evacuated. Other reactive gases such as O₂, CO₂, N₂O, H_(z), H₂Ovapor, and vapors of alcohols (such as methanol, ethanol, isopropylalcohol [IPA]) may be used to further control the UV curing process. Theprocess pressure may vary between 10 to 1000 Torr. At a moderatesubstrate temperature (e.g., between about 25-500° C.) and UV exposure,the H atoms from neighboring broken bonds combine to form H₂ thatdiffuses out of the film. The removal of hydrogen leaves behind microvoids in the film, along with the voids formed during deposition. Thephoton energy from the UV source, coupled with the thermal energy due tothe substrate temperature cause these voids to shrink (in order tominimize surface energy). This shrinkage imposes significant tensilestrains in the film.

The substrate temperature during the UV treatment is affected by thedevice and film on the partially fabricated circuit. The use of nickelmonosilicide (NiSi) layers constrains the substrate temperature to lessthan 400° C., and nickel platinum silicide (NiPtSi), to less than about480° C.

The UV treatment may be implemented in a continuous mode or pulsing modeto further optimize the end result on the final film. In a continuousexposure mode, the film is irradiated with a continuous UV source. In apulsing mode, the film is exposed to pulses of UV radiation, leading toa sequence of curing/quenching/curing/quenching events. By modulatingthe pulse length, the intensity of the pulses, and the spectra of theradiation, the film stress and other properties may be furtheroptimized.

Table 1, below, provides suitable PECVD deposition conditions for asilicon nitride layer suitable as a capping layer in accordance with thepresent invention:

TABLE 1 Parameter Range SiH₄ (sccm) 100-300  NH₃ (sccm) 1000-7000  N₂(sccm) 5000-15000 He (sccm) 2000-10000 HFRF (W) 300-1200 Pressure (Torr)5-15 Temperature (° C.) 250 ≦ X ≦ 480

FIG. 2A shows a plot of the effect of UV curing time and wavelength onthe stress of a PECVD silicon nitride film cured at 400° C. Line 201shows the tensile stress as function of cure time for a broadband UVsource. The film stress saturated at relatively short cure time ascompared to other UV sources, achieving 1.48 GPa. Line 203 shows thetensile stress for a 172 nm source. As discussed above, radiation at awavelength of 172 nm is shorter, and has higher energy, than the Si—Nbond breaking radiation at 225 nm. As expected, the stress alsosaturated after a relatively short cure time, but not as short as thebroadband source. The highest stress was achieved using radiation havinga wavelength of 222 nm, which was up to 1.58 GPa. Even after 120minutes, the stresses appeared to be continuing to increase. Furthercuring may achieve even higher stresses. Line 207 shows the tensilestress for a 308 nm source. The stress obtained was lower, butincreasing. The low stress may be caused by the low intensity ofradiation used in this test (and the low absorption at this wavelength).

FIG. 2B shows a plot of film shrinkage for the same test. As discussedabove, shrinkage is associated with void reduction in the film, andadversely affects hydrogen diffusion in the film. The more a filmshrinks, the less hydrogen would evolve from it as the film becomes morecompacted. Thus, it is desirable to delay the shrinkage until a desiredamount of hydrogen has evolved from the film. Consistent with theshorter cure time to stress saturation of lines 201 and 203, the filmshrinkage of lines 211 and 213 increased to 8% and 7%, respectively, ina short amount of time. Lines 215 and 217, on the other hand, shows amore graduated shrinkage mirroring the tensile stress lines.

The phenomenon of stress saturation and relaxation is shown on FIG. 3A,as lines 301 and 303. Lines 301 and 303 show film stresses on a 700 ÅSi—N film at various cure times under a broadband source. Different UVbulbs were used. At first, the film stresses increased rapidly,consistent with the bond breaking, hydrogen diffusion, and reformationmechanism discussed above. The stresses reached a peak at about 30minutes of cure time in a stress saturation phenomenon and decreasedslowly and the reformed bonds relax the Si—N network. After 120 minutes,the tensile stresses in the film were less than that after about 10minutes of cure time. For broadband sources, longer cure times did notyield higher stresses.

On the other hand, the curves shown for thermal anneal at 500° C. (line305) and 550° C. (line 307) had different slopes from the broadband UVcure. The stresses increased rapidly at first until about 20 minutes ofcure, then increased less rapidly. The stresses continued to increaseeven after 120 minutes of curing, showing no relaxation phenomenon. Thestress obtained at a higher temperature anneal is higher, as evidencedby comparing line 307 to 305. Note however, anneal temperatures of 550°C. and even 500° C. are likely to be above the thermal budget for theunderlying NiSi or NiPtSi layers. Line 309 shows the film stresses overtime for curing at 480° C. with a broadband UV source with a 320 nmlong-pass filter. A long-pass filter allows wavelengths longer than thespecified wavelengths to pass. The 320 nm long-pass filter would allowUV radiation of wavelengths of about 320-800 nm to pass and filter outshorter wavelengths for a broadband source having an output of about170-800 nm. Line 309 has the same slope as that of 307 and 305, boththermal anneals, but a lower substrate temperature than both 307 and305. The stresses obtained at a lower temperature UV cure (line 309 at480° C.) were higher than that of higher temperature thermal anneal(line 305 at 500° C.). Although at the highest stress obtained for thefiltered UV cure did not reach the peak for the broadband cure, the linehad an upward slope and with continued curing would surpass the maximumstress from the broadband UV cure.

FIG. 3B shows the shrinkage percentage for films exposed to variouscuring techniques. Shrinkage was highest for the broadband UV sources(lines 311 and 313) and lowest for the thermal anneal (lines 315 and317). Line 319 shows the shrinkage values for a broadband UV source witha 320 nm long-pass filter. Although line 319 shows higher shrinkagepercentages than that of thermal anneal, the shrinkage rates weresignificantly lower than the broadband lines (311 and 313). The resultdemonstrates that the UV wavelengths shorter than 320 nm, those blockedout by the filter, cause the higher shrinkage.

Test results depicted in FIGS. 2 and 3 show that at a wavelength longerthan 225 nm, UV cure would continue to increase film stresses even afterprolonged curing, after about 120 minutes. The results at wavelengthslonger than 320 nm compares favorably to thermal anneals at highertemperatures. Thus, film stresses may be increased further by limitingthe UV wavelengths to those longer than 225 nm, preferably longer than250 nm.

In order to minimize the shrinkage while driving out the maximumpossible hydrogen, a cascading approach may be used. The cascadingapproach involves sequentially exposing the film to wavelengths above aparticular cut-off wavelength that is increasingly shorter. For example,FIG. 1 shows the absorption of the as-deposited film begins at about 330nm. Exposing the film to wavelengths only above, e.g., 305 nm, willbreak the H terminated bonds, liberating H₂ and generating stress. Inaddition, since this cut-off is far away form the wavelength at whichSi—N starts absorbing, Si—N bond breaking would be minimized. But aftersome time of cure, the absorption curve of the film moves to shorterwavelengths. After this time, there will be no more or very littleabsorption of any radiation that has a wavelength longer than 305 nm.

The next step at that time would be to expose the films only towavelengths above, e.g., 295 nm. Once the film has sufficiently cured atthis wavelength, and the absorption curve has moved below the cut-offwavelength, the film can then be exposed to a lower wavelengthradiation, e.g., 280 nm. This progression can continue until finally thefilm is exposed to radiation at wavelengths near the S—N bond breakingrange, e.g., greater than 225 nm, or preferably grater than about 250nm. The cascaded approach maximizes H₂ removal, while minimizing theamount of time that the film is exposed to high energy (lowerwavelength) radiation which minimizes the matrix bond-breaking andstress relaxation. Although the example above uses cut-off wavelengthsat 305 nm, 295 nm, 280 nm, and 250 nm, the invention is not so limited.Different cut-off wavelengths may be chosen. In some cases, more thanone exposure at the same wavelength or range of wavelengths may occur.

FIG. 4 is an example process diagram for the cascaded approach. Themethod 400 involves providing a heat sensitive substrate (402), such aslayer of silicide. The dielectric layer is formed by depositing adielectric, such as silicon nitride, on the silicide substrate by a CVDprocess, e.g., PECVD, at a temperature of no more than 480° C. (404).Other dielectrics such as silicon carbide, oxygen-doped silicon carbide,nitrogen-doped silicon carbide, silicon boron nitride, silicon boroncarbide, silicon oxide, and combinations thereof may also be deposited.The deposited dielectric is then exposed to UV radiation at a particularwavelength or wavelength range at a moderate temperature, for example,between about 250 and 480° C. (406). The UV radiation may reach thesubstrate through a filter, for example from Hg lamp having a nominalwavelength from about 150 nm to 800 nm, through a long-pass filter for300 nm, resulting in wavelengths of about 300 nm to 800 nm reaching thesubstrate. Alternatively, a non-broadband source may be used thatproduces UV radiation at a particular wavelength or wavelengths.

Table 2, below, provides an example of typical UV curing conditions fora silicon nitride layer suitable as a capping layer in accordance withthe present invention:

TABLE 2 Parameter Range He flow (sccm) 3000-15000   SubstrateTemperature 380-480° C. UV Power (W/cm²) 1.0-3.0    Pressure (Torr)5-200 

In a cascaded approach after the first exposure, a decision is madewhether the desired film stress is achieved (408). After the firstperiod of exposure the film may not absorb further at the longerwavelengths, as evidenced by a shifting of absorption curve (see FIG.1). The wavelength or wavelengths range may be reduced (410) for thenext exposure step at the new wavelength or wavelengths (406). After thedesired film stress is achieved, the process is complete. Note that eachsuccessive exposure operation may be for different durations and atdifferent intensities. The first exposure may be longer at a higherintensity than the second exposure.

In an alternative embodiment, a non-cascaded approach with a frequencycutoff may be taken, as shown in FIG. 5. In this example, the method 500involves providing a heat sensitive substrate (502), such as layer ofsilicide. The dielectric layer is formed by depositing a dielectric,such as silicon nitride, on the silicide substrate by a CVD process,e.g., PECVD, at a temperature of no more than 480° C. (504). Thedeposited dielectric is then exposed to UV radiation at a particularwavelength or wavelength range at a moderate temperature, for example,between about 250 and 480° C. (506). The UV radiation may havewavelengths greater than a cut-off wavelength, e.g., about 200 nm, 225nm, or preferably greater than 250 nm. The particular cut-off wavelengthto use depends on the material being cured. The exposure may be at oneor more wavelength ranges, as long as all UV radiation used are longerthan the cut-off wavelength. This approach avoids unwanted stressrelaxation by reducing UV radiations that would break Si—N bonds;however, the substrate shrinkage may reduce hydrogen diffusion andevolution.

Transistor Architecture

The present invention may be specifically applied to straining an NMOSchannel by inducing stress in a silicon nitride capping layer. As shownin FIG. 6, an example NMOS transistor structure includes a p-dopedsubstrate 602, an n-doped well 604 within the substrate 602, aconductive gate 606 separated from the n-well 604 of the substrate 602by a gate dielectric 608 and p-doped source 610 and drain 612 regions inthe well 604 on either side of the gate 606, and a channel 614 regionunderlying the gate 606. There may also be sidewall spacers 609 on thegate 606. The source 610 and drain 612 regions and the gate 606 arecovered with a layer of self-aligned silicide (salicide) 620, and thesalicide is covered with a silicon nitride (SiN) capping layer 630.

UV curing the capping layer 630 by exposure to UV radiation breaks Si—Hand N—H bonds in the film. Hydrogen is removed from the capping layer,and tensile stress in the capping layer is induced. The tensile stressis transferred to the NMOS channel through the source and drain regionsresulting in a strained channel. As discussed above, channel strainimproves electron mobility and therefore device performance.

Apparatus

The present invention can be implemented in many different types ofapparatus. Generally, the apparatus will include one or more chambers(sometimes referred to as process reactors) that house one or moresubstrates and are suitable for semiconductor processing. At least onechamber will include a UV source. A single chamber may be employed forall operations of the invention or separate chambers may be used. Eachchamber may house one or more substrates for processing. The one or morechambers may maintain the substrates in a defined position or positions(with or without motion within that position, e.g., rotation, vibration,or other agitation) during UV treatment operations. For certainoperations in which the substrate is to be heated, the apparatus mayinclude a heating platen.

In certain embodiments, the cascading cure process, or generally,multi-operation cure process, is performed using a multi-station curechamber. As discussed above, in certain embodiments, the multipleoperation cure processes of the invention rely on being able toindependently modulate the UV intensity, wavelength, spectraldistribution and substrate temperature of each operation. Additionally,certain inert or reactive gases may be injected during the cure processat the same or different flowrates at each step. Multi-station curechambers capable of independent modulation of the above variables aredescribed in U.S. patent application Ser. No. 11/688,695, titled“Multi-Station Sequential Curing of Dielectric Films,” which is herebyincorporated by reference in its entirety for all purposes.

Particularly, the independent control of the substrate temperature andthe UV intensity is described in U.S. patent application Ser. No.11/115,576 and in commonly assigned U.S. patent application Ser. No.11/184,101, now U.S. Pat. No. 7,327,948 filed Jul. 18, 2005, titled“Cast Pedestal with Heating Element and Coaxial Heat Exchanger,” whichare hereby incorporated by reference in their entirety for all purposes.A chamber may decouple substrate temperature and UV intensity byreducing the amount of IR radiation on the substrate, which heats up thesubstrate, and/or providing independent heat transfer mechanisms to andfrom the substrate. For example, the chambers may be equipped with coldmirrors or other reflectors to reduce the amount of IR radiationincident on the substrate, the absorption of which results intemperature increase. In addition, each pedestal or other substratesupport may have an independent heat transfer mechanism to help maintaina substrate temperature regardless of the UV intensity. Thus, unlikeconventional UV cure chambers where substrate temperature is coupled toUV intensity, the substrate temperature and UV intensity may beindependently set for a wide range of temperatures and intensities.

FIGS. 7A and 7B show one embodiment of an apparatus appropriate for usewith certain embodiments of the invention that uses broadband UVsources. Chamber 701 includes multiple cure stations 703, 705, 707 and709, each of which accommodates a substrate. Station 703 includestransfer pins 719. FIG. 7B is a side view of the chamber showingstations 703 and 705 and substrates 713 and 715 located above pedestals723 and 725. There are gaps 704 between the substrates and thepedestals. The substrate may be supported above the pedestal by anattachment, such as a pin, or floated on gas. Parabolic or planar coldmirrors 753 and 755 are located above broadband UV source sets 733 and735. UV light from lamp sets 733 and 735 passes through windows 743 and745. Substrates 703 and 705 are then exposed to the radiation. Inalternative embodiments, the substrate may be supported by the pedestals723 and 725. In such embodiments, the lamps may or may not be equippedwith cold mirrors. By making full contact with the pedestal, thesubstrate temperature may be maintained by use of a conductive gas suchas helium or a mixture of helium and argon at a sufficient pressure forconductive heat transfer, typically between 20 and 760 Torr, butpreferably between 100 and 600 Torr.

In operation, a substrate enters the chamber at station 703 where thefirst UV cure operation is performed. Pedestal temperature at station703 is set to a first temperature, e.g. 400° C., with the UV lamps abovestation 703 set to a first intensity, e.g., 100% maximum intensity, andfirst wavelength range, e.g., about 305-800 nm. After curing in station703 for a sufficient time such that absorption at the wavelength rangeis reduced, the substrate is transferred to station 705 for furthercuring at the same wavelength range or shorter wavelength range, e.g.,295-800 nm. Pedestal temperature at station 705 is set to a secondtemperature, which may or may not be the same as the first station andUV intensity is set to a second intensity, e.g. 90% intensity. Stations707 and 709 may also be used for UV curing. For example, at station 707,the UV radiation may be at a wavelength range of 280-800 nm, and atstation 709, 225-800 nm. Each substrate is sequentially exposed to eachUV light source.

In order to irradiate the substrate at different wavelengths orwavelengths ranges while using a broadband UV source, which generatesradiation in a broad spectrum, optical components may be used in theradiation source to modulate the part of the broad spectrum that reachesthe substrate. For example, reflectors, filters, or combination of bothreflectors and filters may be used to subtract a part of the spectrumfrom the radiation. On reaching the filter, light may be reflected,absorbed into the filter material, or transmitted through.

Long pass filters are interference filters, which provide a sharpcut-off below a particular wavelength. They are useful for isolatingspecific regions of the spectrum. Long pass filters are used to pass, ortransmit, a range of wavelengths and to block, or reflect, otherwavelengths on the shorter wavelength side of the passband. Longwavelength radiation is transmitted, while short wavelength radiation isreflected. The region of high transmittance is known as the passband andthe region of high reflectance is known as the reject or reflectanceband. The roll-off region separates the pass-band and reflect-band. Thecomplexity of long pass filters depends primarily upon the steepness ofthe transition region and also on the ripple specifications in thepassband. In the case of a relatively high angle of incidence,polarization dependent loss may occur. Long pass filters are constructedof hard, durable surface materials covered dielectric coatings. They aredesigned to withstand normal cleaning and handling.

Another type of filter is UV cut-off filter. These filters do not allowUV transmission below a set value, e.g. 280 nm. These filters work byabsorbing wavelengths below the cut-off value. This may be helpful tooptimize the desired cure effect.

Yet another optical filter that may be used to select a wavelength rangeis a bandpass filter. Optical bandpass filters are designed to transmita specific waveband. They are composed of many thin layers of dielectricmaterials, which have differing refractive indices to produceconstructive and destructive interference in the transmitted light. Inthis way optical bandpass filters can be designed to transmit a specificwaveband only. The range limitations are usually dependant upon theinterference filters lens, and the composition of the thin-film filtermaterial. Incident light is passed through two coated reflectingsurfaces. The distance between the reflective coatings determines whichwavelengths will destructively interfere and which wavelengths will beallowed to pass through the coated surfaces. In situations where thereflected beams are in phase, the light will pass through the tworeflective surfaces. However, if the wavelengths are out of phase,destructive interference will block most of the reflections, allowingalmost nothing to transmit through. In this way, interference filtersare able to attenuate the intensity of transmitted light at wavelengthsthat are higher or lower than the desired range.

Another filter that can attenuate the wavelengths of the radiationreaching the substrate is the window 743, typically made of quartz. Bychanging the level of metal impurities and water content, the quartzwindow can be made to block radiations of undesired wavelengths.High-purity Silica Quartz with very little metal impurity is moretransparent deeper into the ultraviolet. As an example, quartz with athickness of 1 cm will have a transmittance of about 50% at a wavelengthof 170 nm, which drops to only a few percent at 160 nm. Increasinglevels of impurities in the quartz cause transmission of UV at lowerwavelengths to be reduced. Electrically fused quartz has a greaterpresence of metallic impurities, limiting its UV transmittancewavelength to around 200 nm and longer. Synthetic silica, on the otherhand, has much greater purity and will transfer down to 170 nm. Forinfrared radiation, the transmittance through quartz is determined bythe water content. More water in the quartz means that infraredradiation is more likely absorbed. The water content in the quartz maybe controlled through the manufacturing process. Thus, the spectrum ofradiation transmission through the quartz window may be controlled tocutoff or reduce UV transmission at shorter wavelengths and/or to reduceinfrared transmission at longer wavelengths.

In addition to changing the wavelengths by altering the radiation thatreaches the substrate, radiation wavelength can also be controlled bymodifying the properties of the light generator. Broadband UV source cangenerate a broad spectrum of radiation, from UV to infrared, but otherlight generators may be used to emit a smaller spectrum or to increasethe intensity of a narrower spectrum. Other light generators may bemercury-vapor lamps, doped mercury-vapor lamps, electrode lamps, excimerlamps, excimer lasers, pulsed Xenon lamps, doped Xenon lamps. Laserssuch as excimer lasers can emit radiation of a single wavelength. Whendopants are added to mercury-vapor and to Xenon lamps, radiation in anarrow wavelength band may be made more intense. Common dopants areiron, nickel, cobalt, tin, zinc, indium, gallium, thallium, antimony,bismuth, or combinations of these. For example, mercury vapor lampsdoped with indium emits strongly in the visible spectrum and around 450nm; iron, at 360 nm; and gallium, at 320 nm. Radiation wavelengths canalso be controlled by changing the fill pressure of the lamps. Forexample, high-pressure mercury vapor lamps can be made to emitwavelengths of 250 to 440 nm, particularly 310 to 350 nm more intensely.Low-pressure mercury vapor lamps emit at shorter wavelengths.

In addition to changing light generator properties and the use offilters, reflectors that preferentially deliver one or more segments ofthe lamps spectral output may be used. A common reflector is a coldmirror that allows infrared radiation to pass but reflects other light.Other reflectors that preferentially reflect light of a spectral bandmay be used. Therefore a substrate may be exposed to radiation ofdifferent wavelengths at different stations. Of course, the radiationwavelengths may be the same in some stations.

In FIG. 7B, pedestals 723 and 725 are stationary. Indexer 711 lifts andmoves each substrate from one pedestal to another between each exposureperiod. Indexer 711 includes an indexer plate 721 attached to a motionmechanism 731 that has rotational and axial motion. Upward axial motionis imparted to indexer plate 721 to pick up substrates from eachpedestal. The rotational motion serves to advance the substrates fromone station to another. The motion mechanism then imparts downward axialmotion to the plate to put the substrates down on the stations.

Pedestals 723 and 725 are electrically heated and maintained at adesired process temperature. Pedestals 723 and 725 may also be equippedwith cooling lines to enable precise substrate temperature control. Inan alternate embodiment, a large heater block may be used to support thesubstrates instead of individual pedestals. A thermally conductive gas,such as helium, is used to effect good thermal coupling between thepedestal and the substrate. In some embodiments, cast pedestals withcoaxial heat exchangers may be used. These are described inabove-referenced U.S. patent application Ser. No. 11/184,101.

FIGS. 7A and 7B show only an example of a suitable apparatus and otherapparatuses designed for other methods involved in previous and/orsubsequent processes may be used. For example, in another embodimentthat uses broadband UV source, the substrate support is a carousel.Unlike with the stationary pedestal substrate supports, the substratesdo not move relative to the carousel. After a substrate is loaded ontothe carousel, the carousel rotates, if necessary, to expose thesubstrate to light from a UV lamp set. The carousel is stationary duringthe exposure period. After the exposure period, the carousel rotates toadvance each substrate for exposure to the next set of lamps. Heatingand cooling elements may be embedded within the rotating carousel.Alternatively the carousel may be in contact with a heater plate or holdthe substrates so that they are suspended above a heater plate.

In certain embodiments, the substrates are exposed to UV radiation fromfocused, rather than, flood lamps. Unlike the broadband sourceembodiments wherein the substrates are stationary during exposure (as inFIGS. 7A and B), there is relative movement between the substrates andthe light sources during exposure to the focused lights as thesubstrates are scanned. In other embodiments, the substrates may berotated relative to the light sources to average out any differences inintensity across the substrate.

Other apparatuses designed for other methods involved in previous and/orsubsequent processes may be used. For example, methods of the inventionmay be used with a standard PECVD chamber used to deposit the precursorlayer if the chamber is equipped with a UV radiation source. Somesupercritical fluid chamber systems may also be configured to include aUV radiation source.

Alternate Embodiments

While the invention has been primarily described and exemplified withrespect to UV exposure of a deposited film, but also applies tosimultaneous UV and thermal and other treatments, or other engineeredmulti-step processes. For example, while it is preferable both from theperspective of process efficiency and effectiveness to deposit thedielectric to be UV cured in a single step, it is also possible todeposit and cure the dielectric in multiple repeating steps to build upa laminate dielectric with increased tensile stress and without theadverse impacts associated with thermal processing. The film stress willchange with respect to the length of the treatment time, UV intensity,UV spectrum, UV operation mode such as pulse and continuous, curingenvironment, film thickness, and substrate curing temperature.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and compositions of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

All references cited herein are incorporated by reference for allpurposes.

1. A method of fabricating a highly tensile dielectric layer on a heatsensitive substrate, comprising: providing a heat sensitive substrate;depositing a dielectric layer on the substrate; exposing the dielectriclayer to ultraviolet (UV) radiation having a first wavelength or firstwavelength range; and repeating the exposing operation one or more timeswith exposure to ultraviolet radiation, the UV radiation in eachexposing operation having a wavelength or a wavelength range, whereinthe wavelength or lower limit of the wavelength range is decreased withsuccessive exposures.
 2. The method of claim 1, wherein during theexposure operations the substrate is maintained at a temperature ofabout 250° C. to 480° C.
 3. The method of claim 1, wherein the repeatingoperation induces a tensile stress in the dielectric layer greater thanthat without the repeating operation.
 4. The method of claim 1, whereinthe dielectric is selected from the group consisting of a siliconnitride, a silicon carbide, oxygen-doped silicon carbide, nitrogen-dopedsilicon carbide, silicon boron nitride, silicon boron carbide, siliconoxide, and combinations thereof.
 5. The method of claim 4, wherein thedielectric is silicon nitride.
 6. The method of claim 1, wherein thefirst wavelength comprises a range of wavelengths.
 7. The method ofclaim 6, wherein the shorter wavelengths each comprise a range ofwavelengths.
 8. The method of claim 7, wherein the first wavelengthrange is about 305-800 nm; a first repeat exposure occurs in awavelength range of about 295-800 nm; a second repeat exposure occurs ina wavelength range of about 280-800 nm; and a third repeat exposureoccurs in a wavelength range of about 225-800 nm.
 9. The method of claim1, wherein the ultraviolet radiation includes only wavelengths longerthan about 200 nm.
 10. The method of claim 1, wherein the ultravioletradiation includes only wavelengths longer than about 225 nm.
 11. Themethod of claim 1, wherein the ultraviolet radiation includes onlywavelengths longer than about 250 nm.
 12. The method of claim 1, whereinthe substrate further comprises an NMOS transistor.
 13. The method ofclaim 1, wherein a long pass filter is used to narrow the wavelengthrange of the ultraviolet radiation emitted by a broadband source. 14.The method of claim 1, wherein a short pass filter and a long passfilter are used to narrow the wavelength range of the ultravioletradiation emitted by a broadband source.
 15. The method of claim 1,wherein the UV radiation is emitted from a UV source comprising a lampwith an emission wavelength range of 150-800 nm.
 16. The method of claim1, wherein the UV radiation is emitted from a UV source comprising alaser or excimer lamp with an emission wavelength of 172 nm, 222 nm, or308 nm.
 17. The method of claim 1, wherein the ultraviolet radiationexposure occurs in a multi-station chamber, each station comprising oneor more UV radiation sources and one or more filters configured toexpose the substrate to one or more UV radiation wavelengths.
 18. Themethod of claim 1, wherein two or more ultraviolet radiation exposingoperations are of unequal duration.
 19. The method of claim 1, whereinthe dielectric layer has a tensile stress is in excess of 1.5E10dynes/cm² or 1.5 Gigapascal (GPa).
 20. The method of claim 1, whereinthe dielectric layer has a tensile stress is in excess of 1.8E10dynes/cm² or 1.8 Gigapascal.
 21. A method comprising: depositing adielectric layer on a substrate; and exposing the dielectric layer toultraviolet radiation having a wavelength longer than a cut-offwavelength; wherein the exposing operation induces a tensile stress inthe dielectric layer greater than that achieved with an ultravioletwavelength range including wavelengths shorter than the cut-offwavelength; and further comprising repeating the exposing operation oneor more times, the UV radiation in each exposing operation having awavelength or a wavelength range, wherein the wavelength or lower limitof the wavelength range is decreased with successive exposures.
 22. Themethod of claim 21, wherein the dielectric is selected from the groupconsisting of a silicon nitride, a silicon carbide, oxygen-doped siliconcarbide, nitrogen-doped silicon carbide, silicon boron nitride, siliconboron carbide, silicon oxide, and combinations thereof.
 23. The methodof claim 22, wherein a long pass filter is used to narrow ultravioletradiation emitted by a broadband source to a wavelength range thatincludes only wavelengths longer than about 250 nm.
 24. The method ofclaim 21, wherein the wavelength and the shorter wavelengths comprise arange of wavelengths.
 25. A method comprising: depositing a dielectriclayer on a substrate; exposing the dielectric layer to ultraviolet (UV)radiation comprising a first wavelength; and repeating the exposingoperation one or more times with exposure to ultraviolet radiation,wherein repeating the exposing operation comprises exposing thedielectric layer to radiation comprising a wavelength shorter than anywavelength used in a previous exposing operation.